IDO1 and cancer: In order for a complete understanding of how immunotherapy is a promising treatment for EBV-associated cancers, we also summarize (a) EBV infection, (b) IDO expression in EBV associated-diseases, (c) EBV regulation of IDO, and (d) concomitantly, of an important immune therapy axis, programmed cell death 1 (PD-1)/ programmed cell death 1 ligand 1 (PD-L1).